Some transmitters within electronic systems include devices (e.g., radio frequency (RF) power amplifiers) that add distortion to a signal along the transmit chain. Such devices are referred to as “non-linear” devices when the added distortion is non-linear in nature. Distortion may include, for example, variations in phase differences and/or variations in amplitude differences. For example, one type of non-linearity occurs when the phase between an input signal and a corresponding output signal varies with input signal amplitude. This is generally referred to as amplitude-to-phase distortion (am/pm). Another type of non-linearity occurs when the ratio of output signal amplitude to input signal amplitude varies with input signal amplitude. This is generally referred to as amplitude-to-amplitude distortion (am/am). Significant distortion, left uncompensated for, may result in poor system performance.
Adaptive pre-distortion (APD) systems have been used in the past to compensate for the intrinsic distortion characteristics of non-linear devices. A traditional APD system includes a feedback path from the system output, which provides a feedback signal reflective of the antenna output signal. The APD system generates an error signal, which reflects differences between the transmitter's input signal and the feedback signal. The error signal is used to determine a complementary distortion (e.g., an inverse gain). The complementary distortion is applied to the input signal to generate a “pre-distorted” signal. When the complementary distortion accurately reflects the non-linear distortion produced in the system, the system may converge to an optimal pre-distorted input signal, in which the non-linear distortion is effectively cancelled.
At times, the feedback signal may include an additional component due to an interfering RF signal that has leaked through the device's antenna and into the feedback path. This interference typically is incoherent, and may appear as an additional noise component on top of the actual output signal. For example, another source (e.g., a proximate wireless device, referred to as a “blocker”) may transmit an RF signal within the same or and adjacent frequency band, which has sufficient power to leak through the device's antenna. Components of the interfering signal that leak through the antenna may combine with and add a noise-like component to the device's output signal. The noise-like component in the output signal is reflected in the feedback signal, and accordingly is reflected in the APD generated error signal and the complementary distortion that is applied to the input signal. This may compromise the convergence of the APD system to an optimal pre-distorted signal, and may also result in poor spectral performance and degraded link quality. Accordingly, a need exists for APD systems and methods that may provide robust convergence to an acceptable pre-distorted signal, even in the face of significant co-channel interference.